The cross-linking of rabbit skeletal muscle sarcoplasmic reticulum protein

Oligomerization of sarcoplasmic reticulum Ca2+-ATPase from rabbit skeletal muscle

Although the primary structure and catalytic cycle of the sarcoplasmic reticulum Ca2+-ATPase has been revealed, it is not well understood whether functional Ca2+ pump proteins exist in a monomeric or an oligomeric state in native skeletal muscle membranes. Here, we show that the Ca2+-ATPase tends to form high molecular weight complexes, estimated to be dimers and tetramers using immunoblotting of two-dimensionally separated microsomal membranes following crosslinking. This agrees with both electron microscopical and biochemical findings which demonstrate that Ca2+-ATPase clusters are the predominant molecular species in native membranes and that oligomerization may play a role in cooperative kinetics and enzyme stabilization.

Purification and oligomeric state of the major lens fiber cell membrane proteins

Purification of the lens fiber cell membrane proteins MP20 and MP26, and the partial co-purification of the lens connexin-related proteins MP70 and connexin 46 has been achieved using anion- and cation-exchange chromatography of lens fiber cell membrane proteins solubilized in n-octyl-beta-D-glucopyranoside (octyl glucoside). The apparent molecular weights of the solubilized protein-detergent complexes were significantly greater than that expected for the monomeric proteins. The purified proteins retained their ability to be phosphorylated by cAMP-dependent protein kinase, and to bind calmodulin in a calcium and magnesium dependent manner. The heterobifunctional covalent chemical crosslinking agent N-5-azido-2-nitro-benzoyloxysuccinimide (ANB-NOS), and the thiol oxidant cupric phenanthroline were used to identify the oligomeric states of these proteins. Crosslinking of either the purified proteins or native lens membranes generated a ladder of crosslinked MP20 or MP26 homo-oligomers. The largest detectable crosslinked homo-oligomer of MP20 was at least a hexamer, while for MP26 the largest crosslinked homo-oligomer was at least a tetramer. The possible oligomeric states of MP70 and connexin 46 could not be determined with the crosslinking reagents used in this study. The procedure described here for the purification of detergent-solubilized major lens proteins should provide a valuable approach in future studies aimed at clarifying the roles of these different lens membrane proteins.

Effect of calcium on the interactions between Ca2+-ATPase molecules in sarcoplasmic reticulum

The interaction between Ca2+-ATPase molecules in the native sarcoplasmic reticulum membrane and in detergent solutions was analyzed by chemical crosslinking, high performance liquid chromatography (HPLC), and by the polarization of fluorescence of fluorescein 5'-isothiocyanate (FITC) covalently attached to the Ca2+-ATPase. Reaction of sarcoplasmic reticulum vesicles with glutaraldehyde causes the crosslinking of Ca2+-ATPase molecules with the formation of dimers, tetramers and higher oligomers. At moderate concentrations of glutaraldehyde solubilization of sarcoplasmic reticulum by C12 E8 or Brij 36T (approximately equal to 4 mg/mg protein) decreased the formation of higher oligomers without significant interference with the appearance of crosslinked ATPase dimers. These observations are consistent with the existence of Ca2+-ATPase dimers in detergent-solubilized sarcoplasmic reticulum. Ca2+ (2-20 mM) and glycerol (10-20%) increased the degree of crosslinking at pH 6.0 both in vesicular and in solubilized sarcoplasmic reticulum, presumably by promoting interactions between ATPase molecules; at pH 7.5 the effect of Ca2+ was less pronounced. In agreement with these observations, high performance liquid chromatography of sarcoplasmic reticulum proteins solubilized by Brij 36T or C12 E10 revealed the presence of components with the expected elution characteristics of Ca2+-ATPase oligomers. The polarization of fluorescence of FITC covalently attached to the Ca2+-ATPase is low in the native sarcoplasmic reticulum due to energy transfer, consistent with the existence of ATPase oligomers (Highsmith, S. and Cohen, J.A. (1987) Biochemistry 26, 154-161); upon solubilization of the sarcoplasmic reticulum by detergents, the polarization of fluorescence increased due to dissociation of ATPase oligomers. Based on its effects on the fluorescence of FITC-ATPase, Ca2+ promoted the interaction between ATPase molecules, both in the native membrane and in detergent solutions.

Cross-linking agents induce rapid calcium release from skeletal muscle sarcoplasmic reticulum

The passive permeability of skeletal muscle sarcoplasmic reticulum vesicles to Ca2+ ions is drastically increased upon addition of the oxidizing agent cupric phenanthroline. The permeability change, which occurs very rapidly, is partially reversed by reducing agents and cannot be explained by a direct effect of cupric phenanthroline on the lipid moiety of the membranes. The rapid efflux phenomenon is due to protein cross-linking induced by the cupric phenanthroline catalyzed oxidation of SH groups to disulfide bridges. Similar effects are also induced by cross-linking sarcoplasmic reticulum proteins with dithiodipropionic acid disuccinimido ester. The rapid Ca2+ efflux is inhibited by micromolar concentrations of lanthanum and by labeling the Ca2+-ATPase with dicyclohexylcarbodiimide. These observations suggest that Ca2+ channels are formed by chemical modification of the ATPase. The Ca2+ permeability rate of sarcoplasmic reticulum obtained after cross-linking is compatible with the requirements of Ca2+ release in vivo. The possibility that Ca2+-ATPase oligomers might mediate the release process is discussed.

The structure of mammalian rod opsins

Ovine rhodopsin is organised in disc membranes as a monomer. The determination of its amino acid sequence has permitted the utilisation of structure prediction programmes which indicate the probable disposition of the polypeptide chain in the bilayer. This putative model is consistent with labelling data using the chemical probes, [14C]succinic anhydride, [125I]diazodiido sulphanilic acid and [125I]iodophenyl azide, and with the cleavage points for several proteases. More surprisingly the predicted structure points to the occurrence of breaks/distortions in the transmembrane helical segments. These distorted regions may be of primary functional importance to the protein and at least one is associated with the attachment point of the chromophore. This particular part of the structure is also identified as a "mutational hot spot", for bovine, equine, ovine and porcine opsins exhibit different sequences (but conserved molecular volumes) in the four residues following the retinyllysine. In an otherwise highly conserved protein with no obvious functional differences between the four species, the high substitution rate in this region is unexplained.

Reconstitution of sarcoplasmic reticulum Ca2+-ATPase with excess lipid dispersion of the pump units

Sarcoplasmic reticulum Ca2+-ATPase has been reconstituted with excess lipid (25-150 g egg phosphatidylcholine per g sarcoplasmic reticulum protein) by a procedure combining the use of a non-ionic detergent with cholate dialysis. The reconstituted vesicles were analyzed by sucrose density fractionation and freeze-fracture electron microscopy. At the lowest lipid to protein ratios some vesicles containing aggregated protein were observed. At a lipid to protein ratio of 150:1 (w/w) only 30-40% of the reconstituted protein sedimented through 7% (w/v) sucrose. The remainder of the latter preparation was characterized by a high Ca2+-uptake capacity and a coupling ratio of 1.6 mol Ca2+ transported per mol ATP hydrolyzed. Intramembranous particles in this preparation occurred isolated in the membrane. In most cases only one particle could be seen on a fracture face. Cross-linking with cupric phenanthroline indicated that protein-protein contacts were drastically reduced by reconstitution. It is concluded that aggregation of intramembranous particles is not required for optimal Ca2+-transport function. The dispersed preparation obtained by a combined reconstitution and sucrose density fractionation procedure is useful for further characterization of the Ca2+ pump.

The structure and organization of dopamine-beta-hydroxylase in the chromaffin granule membrane

Chromaffin granules have been purified from bovine adrenal medullae. The granule membranes have been cross-linked with the disulphide-bridged bifunctional imido ester, dimethyl-3,3'-dithiobissuccinimidylpropionate hydrochloride. Analysis of the cross-linked proteins by electrophoresis on agarose/acrylamide gels revealed components of M(r) 300 000 and 150 000. Further analysis of samples by electrophoresis in a second dimension containing a reducing agent revealed the monomeric species from which the cross-linked polypeptides were formed. The major component in the second dimension exhibited a molecular weight of approx. 80 000 and could be identified with dopamine-beta-hydroxylase (3,4-dihydroxyphenylethylamine ascorbate:oxygen oxidoreductase (beta-hydroxylating), EC 1.14.17.1). It is proposed that dopamine-beta-hydroxylase in the intact granule membrane is arranged as a tetramer consisting of two disulphide-bridged dimers of the 80 000 subunit in close apposition. This structural arrangement of the membrane-bound form of dopamine-beta-hydroxylase is identical with that previously proposed for the soluble, intra-granular form of the enzyme.

A comparison of vesicles derived from terminal cisternae and longitudinal tubules of sarcoplasmic reticulum isolated from rabbit skeletal muscle

Sarcoplasmic reticulum vesicles were separated into heavy (derived from terminal cisternae) and light (derived from longitudinal tubules) fractions, according to Meissner [Biochim. Biophys. Acta, 389, 51-68 (1975)]. The similar Ca2+ sensitivities of phosphoprotein formation, ATPase activity and calcium uptake, and the similar phosphoprotein turnover rates (ATPase/phosphoprotein formation) of both fractions indicate that the same ATPase enzyme is present in the terminal cisternae and longitudinal sarcoplaxmic reticulum. The higher V for Ca2+-activated ATPase activity and calcium uptake in the light fraction correlated with the higher concentration of ATPase enzyme per mg of membrane protein in this fraction. In both the presence and absence of calcium-precipitating anions, the light fraction stored more calcium than the heavy. The Ca2+ dependence of calcium release after addition of EGTA appeared similar in both fractions, but the rate of calcium release was more rapid in the light fraction. These findings suggest that calcium release may occur more rapidly from longitudinal than terminal cisternae portions of the sarcoplasmic reticulum and that calcium release, like calcium uptake, may be mediated by the ATPase enzyme in the sarcoplasmic reticulum membrane. Although the activation energies for Ca2+-activated ATPase activity above and below the transition temperature were significantly different for the heavy and light fractions, their transition temperatures were similar. Partial purification of the ATpase enzyme by deoxycholate treatment modified the activation energies of the light but not the heavy fraction and caused the activation energies to become similar. The phosphoprotein levels of heavy and light vesicles did not become similar after deoxycholate treatment, although gel electrophoretograms indicated both samples contained > 90% ATPase protein. These results indicate the protein-lipid associations in these two fractions may be different.

Crosslinking of sarcoplasmic reticulum ATPase protein with 1,5-difluoro 2,4-dinitrobenzene

The reacton of 1,5-difluoro-2,4-dinitrobenzene with the ATPase protein of rabbit skeletal sarcoplasmic reticulum caused a marked loss of the Ca2+-dependent ATPase (ATP phosphohydrolase, EC 3.6.1.3) activity during an interval when 2 mol of the crosslinking reagent were incorporated/10(5) g of protein. The modified ATPase protein formed non-serial high molecular weight aggregates or oligomers during short (1--5 min) or long exposure (60 min) to the reagent at 25 degrees C or 4 degrees C. The same pattern was found when sarvoplasmic reticulum was treated similarly; only the ATPase protein formed oligomers (homopolymers). In all cases the ATPase protein monomer remained the predominant species present. During the appearance of the high molecular weight oligomers the Ca2+-ATPase activity was unaffected but Ca2+ uptake was inhibited. Major changes in the ATPase activity occurred when the monomeric ATPase protein was modified. Disubstituted dinitrophenylene derivatives of cysteine and tyrosine were found in modified ATPase protein and only a small amount of monosubstituted dinitrophenyl groups were identified. Thiolysis of the modified ATPase protein with 2-mercaptoethanol removed approx. 35% of the incorporated groups, but there was no restoration of the Ca2+-ATPase activity. Substrate MgATP2- protected the Ca2+-ATPase activity of the ATPase protein and sarcoplasmic reticulum but Ca2+ had no effect on the modificaton. Different conformational states of the ATPase protein could be ascertained from a comparison of the effects of Ca2+ and MgATP2- on the bifunctional reagent dinitrophenylation of the ATPOase protein with that of the monofunctional reagent 1-fluoro-2,4-dinitrobenzene (Bailin, G. (1980) Biochim. Biophys. Acta 623, 213-224). Intramolecular crosslinking of the ATPase protein predominated and oligomers which formed during the reaction were not essential for the maintenance of the ATPase activity.

The effects of deoxycholate and trypsin on the cross-linking of rabbit skeletal muscle sarcoplasmic reticulum proteins

Dithiobis (succinimidyl propionate) has been used to cross-link sarcoplasmic reticulum microsome proteins. Although the 100,000 dalton calcium stimulated ATPase and the 60,000 dalton calcium-binding protein calsequestrin were readily cross-linked to form homopolymers, no heteropolymer formation between these two proteins were detected. The 90,000 dalton protein A1 which is always observed in our preparations appeared to preferrentially form dimers on cross-linking. When calsequestrin was solubilized using 0.1 mg deoxycholate/mg protein, this protein was not cross-linked even at dithiobis(succinimidyl propionate) concentrations ten times those used to cross-link this protein in the intact membrane. In a similar manner the deoxycholate-solubilized ATPase (0.5 mg deoxycholate/mg protein) was not cross-linked by dithiobis (succinimidyl propionate). These results suggest that the state of aggregation of the sarcoplasmic reticulum proteins may be modified when solubilized in detergents such as deoxycholate. When the 100,000 dalton ATPase polypeptide was cleaved with trypsin to two fragments with molecular weights of approximately 55,000, these could be readily cross-linked. The fragments were capable of forming polymers with either other 55,000 dalton fragments or with the 100,000 dalton ATPase. The 29,000 and 22,000 dalton fragments, produced by further tryptic cleavage of the 55,000 dalton fragments, were not cross-linked at dithiobis (succinimidyl propionate) concentrations which readily cross-linked the 55,000 dalton fragments. Thus tryptic cleavage of the ATPase to fragments smaller than 55,000 dalton altered associations made by the ATPase in the membrane.